Until recently, laterally diffused Metal-Oxide-Semiconductors (LDMOS) were the preferred technology for amplifying large powers up to around 3 GHz, generally in a quasi-linear operating mode (class AB). LDMOS is typically used due to satisfactory cost/performance trade-offs.
However, these types of devices suffer from efficiency issues, and are not compatible with very fast switching signals, due to a limited transition frequency.
Recently, technological improvements within the semiconductor industry have resulted in increased use of switched mode power amplifiers with increased functionality, allowing them to operate at high power and switching frequencies for use with cellular telecommunications.
Referring to FIG. 1, a known transmitter 100, as illustrated in WO2005120001, utilises a digital band-pass sigma—delta (ΣΔ) modulator 40 to up-convert the baseband signal to an intermediate frequency, apply a digital finite impulse response filter, and drive an array of MOS switches. Digital baseband (DBB) input signals are received by a cellular digital signal processing function 35, which performs serial-to-parallel conversion, digital filtering, splitting of the signals into in-phase and quadrature-phase components and sample rate conversion. The in-phase component of the signal is forwarded to in-phase digital up-converter function 36I, and the quadrature-phase component of the signal is forwarded to quadrature-phase up-converter function 36Q. The outputs of in-phase and quadrature-phase digital up-converter functions 36I, 36Q are applied to inputs of adder 38, which combines the up-converted components and applies the result to digital band-pass sigma-delta modulator 40. The modulated output signals from digital band-pass sigma-delta modulator 40 are filtered by finite impulse response (FIR) digital filter 42, and are the input signals applied to MOS power switch array 44.
Digital band-pass sigma-delta modulator 40 converts the relatively wide input data into a fewer number of bits per sample, having a frequency spectrum that is centred at a desired transmit frequency. Further, digital band-pass sigma-delta modulator 40 is constructed to have notches, or ‘zeroes’, on either side of the desired transmit frequency, with at least one of the notches corresponding to the centre of the receive band.
Class D power amplifiers, in theory, can achieve very high power efficiencies, due in part to these types of amplifiers utilising a square wave, resulting in a unity peak-to-average power ratio (PAPR). However, generally, these types of power amplifiers have typically been used to amplify base-band signals, for example as used in audio amplifiers. The application of switched mode (e.g. Class D′) amplifiers for cellular communications, such as orthogonal frequency division multiplex (OFDM) and wideband code division multiple access (WCDMA) communication systems, has been problematic, not least because the signals to be amplified are at the desired radio frequency (RF). Furthermore, the data rates to be supported approach the channel capacity and the modulations employed to achieve such performances typically require a large crest factor (namely the ratio between the peak-power at a given occurrence and the root mean square (RMS) power). Even with a help of an efficient crest factor reduction algorithm, the typically modulation schemes require significant back-off of the power amplifier from the optimum operating point, which greatly reduces the efficiency and level of average power transmitted.
The switch mode power amplifier may, in theory, achieve very high power efficiencies, since the input signal is a square wave with an unity peak to average power ratio (PAPR). Thus, no back-off of the power amplifier from the optimum operating point is required. However, for cellular operation at the desired RF frequency, these types of class D power amplifiers can suffer from very high sampling frequencies and lower output efficiencies when utilised for RF power amplification. As such, their use has, thus far, been limited.